The present invention relates to an optimized method for transferring an useful layer of monocrystalline silicon carbide (SiC) derived from a source substrate of the same material to a receiving or stiffening substrate. The method permits recycling of the substrate after the transfer of the thin useful layer.
The method known under the trademark “SMART-CUT®” enables a thin layer to be transferred from a source substrate to a receiving substrate such as, for example, an oxidized silicon or polycrystalline silicon carbide substrate. This method also enables the source substrate from which the thin layer has been taken to be reused. However, after each layer transfer step, the upper surface of the source substrate has a certain number of surface irregularities. The formation of these surface irregularities will be described with reference to FIGS. 1-5, which show a specific example of implementation of the “SMART-CUT®” method. This method is known to those skilled in the art and will not be described in detail.
FIG. 1 shows a source substrate 1 which has a planar face 2, or a “front face”, through which gas species have been implanted. This implantation is carried out by ion bombardment, for example with H+ ions (reference B in FIG. 1) using an implanter. The implantation is performed at a specific energy, implantation dose, and temperature, and creates a weakened zone 3 in the neighborhood of the mean implantation depth p of the ions. This weakened zone 3 delimits two portions in the source substrate 1: an upper thin layer or useful layer 100 extending between the front face 2 and the weakened zone 3, and the remainder 10 of the substrate. As shown in FIG. 2, a stiffener or receiving substrate 4 is then applied to the front face 2 of the source substrate 1. This stiffener is chosen by a person skilled in the art as a function of the final application envisaged. The receiving substrate 4 may be applied in a known manner, for example, by evaporation, spraying, chemical vapor deposition, or it can be bonded to the front face by using an adhesive or by a technique known as “bonding by molecular adhesion”, also known as “wafer bonding”.
As shown in FIG. 3, the thin layer 100 is detached from the remainder 10 of the substrate 1. This detachment step, which is symbolized by the arrows S, may be performed by applying either mechanical stresses to the stiffener 4, or by the applying thermal energy to the assembly that includes the stiffener 4 and the substrate 1.
Wafers chosen for use as a source substrate 1 possess reduced edges due to chamfering operations performed, for example, during their manufacture. As a result, the adhesion forces between the stiffener 4 and the front face 2 are weaker in a substantially annular periphery of the source substrate 1. Consequently, when the stiffener 4 is detached from the source substrate 1, only the central portion of the thin layer 100, which is strongly adhering to the stiffener 4, is detached, while the substantially annular periphery of the useful layer 100 remains attached to the remainder 10 of the source substrate 1 as shown in FIG. 3. As a result, the source substrate 1 thus simultaneously includes a surface roughness 11 in its central portion due to detachment in the region of the weakened zone 3, and at its periphery has an excess thickness 12 or surface topology in the form of a blistered zone corresponding to the zones that were not transferred to the receiving substrate or stiffener 4. The depth of this excess zone 12 is equal to the thickness of the transferred thin layer 100. It typically varies from several tens of nanometers to more than a micrometer. The depth is determined by the implantation energy of the hydrogen ions.
In FIGS. 3 and 4, the excess zone 12 has intentionally been shown, for the sake of clarity and simplification, having a rectangular cross section and having a noticeable thickness with respect to the remainder 10 of the source substrate. In reality, it has a much more irregular shape and a proportionally smaller thickness.
Before proceeding to transfer another thin layer, it is imperative to recycle the remainder 10 of the source substrate. This recycling consists of a planarization step, depicted by the arrows P in FIG. 4, wherein the excess zone 12 is eliminated, and a specific finishing step depicted by the arrow F in FIG. 5, that permits elimination of the surface roughness 11 to attain a substrate having a new front face 2′. These recycling steps are generally performed by mechanical and/or mechanical-chemical polishing techniques. In the specific case where the source substrate 1 is made of silicon carbide, an extremely hard material, such polishing steps are extremely long and costly.
The prior art document, “The effects of damage on hydrogen-implant-induced thin-film separation from bulk silicon carbide”, R. B. Gregory, Material Research Society Symposium, Vol. 572, 1999, discloses that the choice of hydrogen implantation conditions for implanting into silicon carbide permits varying the percentage of removal of the excess zone, that is, the percentage of the free surface of silicon carbide which is spontaneously eliminated during thermal annealing of the substrate. In this article, the results show the H+ ion implantation dose as a function of the percentage of the excess zone removed is a bell-shaped curve at an implantation energy of 60 keV, with a maximum value of 33% of the zone removed, for an implantation dose of 5.5×1016 H+/cm2. When departing from this value, that is if the implantation dose is increased or decreased, then the removal percentage decreases.
The document “Complete surface exfoliation of 4H—SiC by H+ and Si+ co-implantation”, J. A. Bennett, Applied Physics Letters, Vol. 76, No. 22, pages 3265-3267, May. 29, 2000, describes that it is possible to perform a complete exfoliation of the surface of a silicon carbide substrate by co-implanting H+ and Si+ ions. More specifically, this document describes tests performed on 4H—SiC silicon carbide by implanting Si+ ions at various doses and at an energy of 190 keV, then implanting H+ ions at an implantation dose of 6×1016 H+/cm2 and at an energy of 60 keV. Implantation doses of Si+ greater than or equal to 5×105 Si+/cm2 permitted exfoliation of 100% of the silicon carbide surface. However, the Si ion implantation dose necessary for total exfoliation of the SiC layer is also high enough to render the silicon carbide amorphous. This method is therefore incompatible with transferring a thin film of silicon carbide of good crystalline quality, since it is not possible to utilize a thin film from such a substrate for forming devices used in microelectronics or opto-electronics.